Harry V. Jaffa, an Abraham Lincoln scholar and political philosopher who made the ideas of America's founding fathers a cornerstone of modern conservative thought, died Saturday at Pomona Valley Hospital Medical Center. He was 96.

His death from natural causes was confirmed by his son Philip.

Jaffa, who taught political philosophy at Claremont McKenna College for 25 years, cemented his reputation as an intellectual force with his 1959 book "Crisis of the House Divided," a seminal interpretation of the Lincoln-Douglas debates that illuminated the classical roots of Lincoln's political principles.

During a brief involvement with electoral politics Jaffa also crafted one of the most memorable campaign lines in modern history: 1964 presidential candidate Barry Goldwater's declaration that "extremism in the defense of liberty is no vice" and "moderation in the pursuit of justice is no virtue."

Those words—lifted by Goldwater's speechwriters from a memo Jaffa wrote—are widely credited with galvanizing the arch-conservative senator's most devoted supporters who, after his lopsided loss to President Lyndon B. Johnson, carried on the movement that later swept Ronald Reagan into the White House.

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FOR THE RECORD: A previous version of this article quoted Barry Goldwater as saying "extremism in pursuit of liberty is no vice" and "moderation in defense of justice is no virtue." Also, Walter Berns' first name was misspelled as Walters.

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Jaffa did not believe Goldwater could win but supported the campaign, he told New York magazine in 2012, as an opportunity "to educate the American people and the conservative movement itself, which I hoped to influence."

Decades later, Jaffa was hailed as a prophet of a brand of American conservatism that could trace its lineage to Lincoln, Thomas Jefferson and earlier proponents of natural law, the idea that human conduct should be guided by a universal standard of justice rather than the arbitrary rules of society.

"Harry helped to reshape the American conservative movement over the course of his long life," said Charles R. Kesler, a Claremont McKenna professor of government. "He engaged in a series of important debates with very prominent members of the conservative movement. He won more than he lost. And the effects were salutary," helping to diminish the influence of ultra-right factions pushing hardline positions on states' rights, immigration and other hot-button issues.

"If you think Harry Jaffa is hard to argue with, try agreeing with him," William F. Buckley Jr., the conservative pugilist who often found himself a subject of Jaffa's ire, once wrote. "He studies the fine print in any agreement as if it were a trap, or a treaty with the Soviet Union."

Jaffa acknowledged there was some reality behind Buckley's witty critique.

"I think it is true, with some possible exceptions ... that everything I have written about conservatism has been an attack on conservatives," the Claremont scholar said in 1984 at a dinner for his 65th birthday. "I don't think I'm particularly contentious. I just show a concern for the truth."

As evidence of Jaffa's impact, Kesler pointed to National Review, the conservative journal founded by Buckley. It recently said: "Jaffa may be the most important conservative political theorist of his generation. … Modern conservatism's focus on the Declaration of Independence and the Constitution—never absent, but increasingly prevalent in these tea-party times—owes much to Jaffa and his circle."

Jaffa died on the same day as Walter Berns, 95, a leading neoconservative and constitutional scholar at Cornell University whose views he had often pummeled.

"I do not mean to be gentle with you," Jaffa once wrote in an open letter to Berns. "In your present state of mind, nothing less than a metaphysical two-by-four across the frontal bone would capture your attention."

Berns' reply was blunt: "Who will rid us of this pest of a priest?"

Born Oct. 7, 1918, in New York City, Harry Victor Jaffa was the eldest of two children of Arthur Jaffa, a restaurateur and nightclub owner, and his wife, Frances.

Jaffa was determined to go to Yale after he was warned that Jews like him would not be accepted. He entered the prestigious university at 16, graduating in 1939 with a degree in English. He taught at Queens College, City College of New York and the University of Chicago before earning a doctorate from the New School for Social Research in 1951.

At the New School he studied under German philosopher Leo Strauss, whose ideas shaped the neoconservative movement as well as the direction of Jaffa's career.

As a graduate student in 1946 Jaffa was browsing the stacks in a second-hand bookstore in Greenwich Village when he stumbled across a copy of the 1858 debates between Illinois Sen. Stephen A. Douglas and his Republican challenger from Springfield. Jaffa was captivated by the debates, he told Forbes' Peter Robinson in 2009, "because they were such wonderful reading."

But because he was at the same time also reading Plato's Republic with Strauss, he came to realize that the Lincoln-Douglas debates were much more than a good read: They were a Socratic dialogue on right versus might.

"Douglas's doctrine of 'popular sovereignty' meant no more than that: in a democracy, justice is the interest of the majority, which is 'the stronger,' " Jaffa later wrote in "Crisis of the House Divided," his classic book on the 16th president. "Lincoln, however, insisted that the case for popular government depended upon a standard of right and wrong independent of mere opinion and one which was not justified merely by the counting of heads."

More than 40 years after "Crisis" was recognized as a landmark work of Lincolnian scholarship, Jaffa produced a sequel, "A New Birth of Freedom: Abraham Lincoln and the Coming of the Civil War" (2000). His other books include "Shakespeare's Politics" (with Allan Bloom, 1964), "American Conservatism and the American Founding" (1984) and "Crisis of the Strauss Divided" (2012), a collection of essays on his influential teacher

Jaffa taught at Ohio State University for 13 years until 1964. That year he moved to Claremont McKenna College and Claremont Graduate University, where he was the Henry Salvatori professor of political philosophy. In 1989 he became an emeritus professor and joined the Claremont Institute.

His wife of 68 years, Marjorie, died in 2010. Besides his sons Philip of Arlington, Va., and Donald of Twentynine Palms, he is survived by daughter Karen Jaffa McGoldrick of Alpharetta, Ga., and three grandchildren.